ABSTRACTUnderstanding the pathomechanisms that lead to accumulative protein aggregation in neurodegenerative disorders poses a major challenge in the field. Tauopathies are no exception. Tauopathies encompass a large, diverse group of neurodegenerative disorders characterized by the aberrant aggregation and accumulation of tau protein, the eponym of these conditions, in the brain. Alzheimer’s disease (AD) represents the most common tauopathy and the leading cause of dementia worldwide. Other tauopathies include progressive supranuclear palsy, corticobasal degeneration, chronic traumatic encephalopathy, and frontotemporal dementia. Although abnormal tau aggregates unify all tauopathies as the major pathological hallmark, they markedly differ in the affected brain regions, cellular lesions, type of tau aggregates, and, hence, clinical presentations. Pathologically, tau undergoes aberrant conformations and folding promoting its aggregation into dimeric, trimeric, and oligomeric structures. Oligomeric tau aggregates further form filaments that ultimately coalesce into higher order ultrastructure e.g., neurofibrillary tangles in AD. Mounting evidence has confirmed that the accumulation of early pretangle oligomeric tau push neurons over the precipice to toxicity. On the other hand, the formation of neurofibrillary tangles may indicate a neuroprotective pathway against cellular demise. Indeed, demystifying the molecular mechanisms that underlie abnormal sequential aggregation events of tau is paramount to develop effective therapeutic strategies. Extensive research over the years has pinpointed some factors that could play a role in the biogenesis of pathological tau aggregates. Tau-related factors such as post-translational modifications, mutations, and truncations are proposed to potentially impact pathological tau aggregation. Furthermore, a growing interest has been directed to investigate tau-interacting proteins and the role they play in the pathological trajectory. Some tau interactors may induce the accumulation of oligomeric aggregates whereas others inhibit tau aggregation. Our group discovered EFhd2 as a tau-associated protein in a transgenic model of tauopathy and postmortem tauopathy brains. EFhd2 is a calcium-binding protein that is highly expressed in the central nervous system. Still, the physiological function of EFhd2 in the brain remains poorly understood. Through several studies, we showed that EFhd2 interacts with tau and promotes its aggregation by altering its dynamic properties in vitro. Based on our previous findings, a follow-up question that remains unanswered is ”What is the role of EFhd2 in tau pathology?” Herein, we utilized a multidisciplinary approach to answer that question. In particular, we first examined the impact of the recombinant human EFhd2 on monomeric and filamentous recombinant human tau in vitro (Chapter Two). The most striking observation was the ability of EFhd2 to entangle in vitro-formed tau filaments into larger aggregates without influencing filament formation. It is important to note that this key observation has not been reported for other tau-interacting proteins. Furthermore, using mass spectrometry analysis, we investigated proteome changes in the brain of our novel Efhd2-/- mouse model (Chapter Three). Hence, we determined the biological pathways mostly affected by the absence of EFhd2 highlighting the potential physiological significance of EFhd2 in the brain. In the same study, we explored the unstudied brain EFhd2 interactome. Indeed, EFhd2 interactome network and proteome changes in Efhd2-/- mice brain underscore the possible, indirect role of EFhd2 in tau pathology and other neurodegenerative diseases. Lastly, we examined the impact of deleting Efhd2 gene in vivo on the progressive pathological phenotype and neuropathological changes of tau in TauP301L expressing mice by developing the TauP301L/Efhd2-/- mouse model (Chapter Four). The results revealed that the absence of EFhd2 induced a moderate-large increase in pretangle oligomeric tau conformations accompanied by a reduction in later tangle markers. In conclusion, those three interconnected studies complement our previous findings on the association of EFhd2 with tauopathies. The presented data provide cogent evidence to the ability of EFhd2 to modulate the biogenesis of tau aggregates promoting the formation of higher order tangle structures. Moreover, we gained new insights into the possible, multifaceted role of EFhd2 in neurodegeneration, provoking a number of future studies. In essence, this research lays the groundwork to determine the significance of EFhd2 as a therapeutic and/or diagnostic target in neurological disorders, especially tauopathies.
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